It is an extraordinarily difficult problem to transform scalar code into vector instructions. The only way to get even passable output from a vectorizing compiler is to write the code as vectors to begin with, such as with cross-platform assembly tools like Orc.
And even then you'll often end up significantly worse off than if you wrote the assembly by hand.
A run of Intel's compiler on the C versions of our DSP functions resulted in a grand total of one vectorization, which was done terribly, too.
